SoilSurvey Information for Community Development In Arizona
WHO USES SOIL SURVEYS
Soil surveys available from the Natural Resources ConservationService (NRCS) are intended for many different users. They canhelp a home buyer or developer determine soilrelated hazards or limitation that affect homesites. They canhelp land use planners determine the suitability of areasfor housing or onsite sewage disposal systems. They can help afarmer estimate the potential crop or forage production ofhis land. They can be used by civil engineers to determinethe suitability and limitations of soils for pipelines,buildings, landfills recreation areas, and many other uses. Thispage explains just a few of the ways soil surveys can help landusers for community development in Arizona.
WHAT ARE SOIL SURVEYS
Natural Resources Conservation Service soil scientists study,evaluate, classify, and map soils in Arizona and throughout thecountry. Soil maps and descriptions of soils are published insoil surveys. The soil boundary lines shown on soil maps can berelated to local plat maps to identify the kinds of soil in givenareas.
Soil maps and supporting data provide information aboutimportant soil properties, including the following: flood hazard,wetness, erodiability, bearing capacity, depth to bedrock, andshrink swell.
Are all Soils More or Less Alike?
Soil scientist have identified over 6,000 unique anddistinctly different soils in Arizona. Most soils consist ofthree distinct layers: surface layer, subsoil, and parentmaterial or bedrock.
Each soil has unique physical and chemical properties. Theseproperties interact. By studying the interaction, soil scientistsand other professionals can predict how a soil will respond tospecific land use and management.
Why SoilSurvey Data Are NeededFor Community Development
Many people assume that soils are all more or less alike. Theyare unaware that great differences in soil properties can occurwithin even short distances.
Soils may be seasonally wet or subject to flooding.They may be shallow to bedrock. They may be too unstableto be used as a foundation for buildings or roads.Very clayey or wet soils are poorly suited to septic tankabsorption fields. A high water table makes a soil poorlysuited to basements or underground installations. These soilproperties and many others that affect land use are given in soilsurveys. Each soil survey describes the properties of soils inthe county or area surveyed and shows the location of each kindof soil on detailed maps.
Like any tool, a soil survey is helpful only if you know whatit can and can’t do -- and if you use it accordingly. Thesurvey does not replace careful onsite investigation or analysisby a soil engineer or other professional
EvaluatingAreas for Houses and other Buildings
Soil properties are a major consideration for all buildingconstruction. The range of potential soil related problems isgreat. Swelling and shrinking of certain kinds of clayey soilsmay crack walls and foundations. Flooding or high water tablesmay flood basements or damage buildings and other installations.Soils that are too clayey or too wet are not suitable for septictank absorption fields. These and other soil related problems canbe anticipated through the use of soil surveys.
Soil surveys can help in evaluating the suitability of a tractof land for the intended use before buying. Where soil maps showthat soil related hazards may damage structures or installations,alternate sites that have favorable soil properties can beselected or structural designs can be changed to compensate forthe hazards. Soil maps and descriptions of the soils also canhelp in planning development in accordance with soil capabilitiesand limitations.
Soil properties are a major consideration in selecting andplanting trees, shrubs, and soil related hazards may damagestructures or installations, alternate sites that have favorablesoil properties can be selected or structural designs can bechanged to compensate or the hazards. Soil maps and descriptionsof the soils also can help in planning development in accordancewith soil capabilities and limitations.
Soils vary in their resistance to erosion, but erosion isgenerally worse in sloping areas. Soil surveys indicate theerodibility of each mapped soil.
Bare, eroded areas detract from a home’s appearance, anderosion can have serious consequences. Erosion along thefoundation of a house can eventually undermine it. Gullies mayform. Mud from eroded areas can build up on your driveway orsidewalk or in other places.
When surface runoff carries eroded soil into a local stream,the sediment can damage water quality, clog reservoirs, andincrease the chance of downstream flooding.
The best defense against erosion is to control runoff and toprotect the soil with grasses, shrubs, and other adapted plants.The soil survey indicates the suitability of each mapped soil forlawn and landscaping plants.
Erosion can be severe on construction sites where the soil isbulldozed and cleared of vegetation. Mulch, sediment traps, and atemporary cover of fast growing grasses help to control erosionon construction sites. Natural Resources Conservation Servicepersonnel can provide information on these and other measures forcontrolling erosion on homesites.
A site erosion control plan or conservation plan is requiredby many Arizona communities before construction is authorized.Soil surveys provide the information needed to prepare aneffective conservation plan based on the soil limitations andhazards in the developing area. Soil maps and information canhelp you plan measures to reduce erosion and sedimentationincident to development and adapt lots and streets to the naturalterrain and soil conditions. They also can help you identifyareas where measures to control runoff and insure adequatedrainage are needed.
Because of rapid suburban expansion, the number of homes thatdo not have access to a public sewage disposal system hasincreased greatly. The most common system for individual homes isone in which the sewer line from the house leads to anunderground septic tank in the yard. Overflow from the tankdisperses into the soil through a system of underground drains orperforated pipes.
To design a system that will work, you need to know thecapacity of the soil to absorb effluent. Movement of effluentthrough soil is determined mainly by the porosity of the soil,the size of the soil particles, and by the kind of clay in thesoil. Effluent moves faster through sandy and gravely soils thanclayey soils. Soils high in clay content have limited pore spacefor holding effluent. Some kinds of clay expand when wet andclose the pores entirely. Such soils are unsuitable forabsorption fields. If the soil is not porous, the effluent simplybuilds up and seeps to the surface.
Soils that have a high water table may be saturated part ofthe year. A saturated soil cannot absorb additional liquid, andthe unfiltered septic tank effluent discharged into drains mayseep to the surface. If there is a seasonal high water table, theseptic tank absorption field may work in dry seasons but fail inwet seasons.
Soil that is shallow to rock or soil that has a cemented layerjust below the bottom of the trench in which drains are laidlacks space for the effluent to be absorbed. About 4 feet may berequired if the underlying rock contains water used for domesticuse.
Steep slopes -- 15 percent or steeper -- make it difficult tocontrol the distribution of effluent. Effluent distributed intosoil on a steep slope may seep onto the ground surface at a lowerlevel. Digging drain trenches on the contour insures that theeffluent flows slowly through the drains and disperses throughoutthe absorption field.
Soils that are subject to flooding should not be used forabsorption fields. Flooding destroys the effectiveness of thefield and allows unfiltered effluent to pollute the stream.
Should YourHome Have A Special Design?
The soil survey does not replace careful onsite investigationor analysis by a soil engineer or other professional. Many soilhazards can be overcome by special structural design orinstallation. Homebuyers or homeowners who are aware of potentialhazards can obtain assistance from consultants for specialinvestigations and designs.
Where do I Get Help?
Civil Engineer Consultants
Health Departments
City & County Planning and Zoneing Departments
Arizona Department of Environmental Quality
Where Do I Get Soil SurveyInformation?
For questions about soil survey availability in Arizona callRob Wilison, Natural Resources Conservation Service, SoilSpecialist at 602-xxx-xxxx.
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